1. Field of The Invention
The present invention relates to thermoplastic or thermosetting resin composite material incorporating an improved reinforcement fabric. More particularly, the invention relates to the use of an improved glass and graphite composite reinforcement to impart anisotropic properties a shaped composite structure. The resulting composite structure, such as an arch support for a shoe, is strong, lightweight and resilient with improved flexing properties.
2. Description of Related Art
During the past several years many researchers working with structural materials have focused on developing strong lightweight articles that are durable, cost effective and easy to fabricate. Such materials would be in great demand for applications ranging from aircraft construction to athletic equipment. For example, structural materials having high durability and significant strength to weight ratios are necessary for many advanced aerospace applications. Similarly, low weight resilient materials with shock absorbing flexibility are continually in demand for fabricating equipment and structural supports. Traditional fabrication materials such as metal alloys and plastics have not proved satisfactory in providing the desired combination of properties. For instance, if acrylics or other plastics are used to form articles they must sacrifice low weight and flexibility in order to achieve the strength necessary for many applications.
The search for substances having these desirable properties has resulted in the extensive development of composite materials. Composite materials are materials in which two or more distinct substances such as metals, glass, ceramics, or polymers are combined, with or without chemical reaction, to produce a material with structural or functional characteristics different from the individual constituents. The constituents retain their individual characteristics and are distinguishable on a microscopic scale. Typically one constituent is classified as the reinforcement and the other as the matrix. The reinforcement provides the strength or stiffness in the composite. The matrix binds the reinforcement together and contributes to the distribution of the load.
To a greater or lesser extent composite materials usually require relatively more effort for their fabrication. Yet despite the complications inherent in their preparation, composite materials represent an interesting alternative to metals whenever there is a demand for great strength with minimal weight. Other than metal alloys, this is only attainable with materials having high tensile strength and low density.
Classes of materials commonly used for reinforcements are glasses, metals, polymers, ceramics and graphite. The reinforcement can be in many forms, such as continuous fibers or filaments, chopped fibers, woven fibers, particles or ribbons. The criteria for selecting the type and form of reinforcement will vary in accordance with the design requirement for the composite. However, criteria for a generally desirable reinforcement include high strength, high modulus, low weight, low cost, ease of fabrication and environmental resistance. The properties of the composite material are derived from matrix characteristics in combination with the inherent properties of the reinforcement material along with the form and amount of reinforcement used. Composite materials typically incorporate several layers or laminae of reinforcing material into a composite structure or laminate.
The prior art contains numerous examples of different materials having these criteria to a greater or lesser extent being employed as reinforcements in composite structures. Those reinforcement materials which have generally favorable properties confer elastic rigidity, tensile and fatigue strength, as well as appropriate electrical and magnetic properties to the resulting composite. The basic problem with current composite reinforcement materials is that they fail to provide all the desired attributes simultaneously. Thus the properties of impact absorption, variable flexibility, ease of fabrication, cost and durability are often mutually exclusive in existing composite materials.
Though an endless number of reinforcement materials may be employed to satisfy different structural or functional requirements, relatively few are extensively used. Due to their low cost and reproducibly good properties, glass fibers have become one of the principal reinforcement materials in use today. The glass fibers are prepared by melting raw materials and extruding the molten glass to yield an amorphous, anisotropic product. Along with their low cost, glass fibers generally have a high strength to weight ratio but their moduli are significantly lower than those of most other high performance fibers. Therefore they may be used to fabricate materials which are relatively flexible. Several types of specialized glass with selected properties have been developed for use in composite materials. Of the glass fibers typically found in reinforcing materials, E-glass is the most common grade and has the lowest cost per unit.
Carbon fibers are currently the predominant high strength, high modulus reinforcing fiber used in the manufacture of advanced composite materials. Production methodology can increase the extent of crystallite orientation parallel to the carbon fiber axis and thus increase the fiber modulus. Because of the high degree of internal structure orientation, the graphite fibers are strongly anisotropic. Their transverse tensile strength and sheer moduli are usually an order of magnitude lower than the axis modulus. Although carbon fibers have been produced with diameters in excess of 25 .mu.m, most fibers are on the order of 6-8 .mu.m in diameter. With such small sizes the carbon filaments must be handled as tows rather than individual microfilaments. Commercially available tows contain anywhere from 1,000 to 60,000 fibers per yarn.
As indicated previously a lamina is defined as one layer or ply of reinforcement material embedded in the matrix. The properties of each lamina are determined by the properties of its constituents as well as the form, orientation and amount of reinforcement used. In general laminae employing long continuous fibers running parallel to each other are stronger than those using short, randomly oriented fibers. Such laminae are anisotropic in that they are stronger and stiffer along the longitudinal axis running parallel to the fibers than the transverse axis running perpendicular to the fibers. In addition, laminae incorporating woven reinforcements are generally stronger along the transverse axis than those with unwoven parallel fiber reinforcements.
The prior art teaches that the laminae may be combined to form laminate structures having properties determined by the orientation of the reinforcement material in the laminae. To compensate for the low transverse properties of the unidirectional material, laminae may be cross plied so the fibers are angled relative to each other. This tends to give structures with improved transverse properties but at the expense of poorer longitudinal properties. Furthermore the in-plane shear strength is not significantly improved over that of unidirectional structures. Thus if the laminate is not constructed so it is balanced and symmetric, it will twist or bend when in-plane loads are applied. The laminate may also extend or contract when bending loads are applied.
Despite these limitations, thermosetting laminate materials have long been used to provide complex shapes in articles of manufacture. For example U.S. Pat. No. 4,439,934 discloses the use of layered materials to form a laminate orthotic insert. The manufacturing process consists of laboriously combining layers of fibers at different angles to provide the strength and flexibility required by the article. Labor intensive, this fabrication method is highly susceptible to manufacturing defects. Construction of the layered article is done on a cast and the whole combination is thermally set to fix the configuration. The resulting insert is relatively thick and heavy with little flexibility for the comfort of the user.
Another example of using a multilayer laminate system may be found in U.S. Pat. No. 4,688,338. This patent teaches a laminated structure providing a greater resistance to bending moments along the longitudinal axis and less resistance to bending along the transverse axis. Yet these beneficial properties are imparted by the interaction of separate, resin impregnated laminae having parallel reinforcement fibers embedded in the matrix. The anisotropic flexibility is imposed solely through the interaction of different layers having the parallel reinforcement fibers oriented at specific angles relative to each other. There is no teaching that one lamina could retain this anisotropic flexibility through the use of a fabric reinforcement layer.
In addition to the reinforcement materials, the other major component of any composite material is the matrix. The matrix binds the reinforcement together and enhances the distribution of the applied load within the composite. Polymeric materials are widely used as matrix materials. The two general types of polymers which are generally employed in composite materials may be classified as thermosetting and thermoplastic. The principal differences between them is the degree of crosslinking and response to elevated temperature. Thermosetting resins or polymers are extensively crosslinked and undergo irreversible changes when heated or reacted with a selected catalyst or a curing agent. In contrast thermoplastic materials are generally not crosslinked and soften as they are heated. After being exposed to heat they return to their original condition when cooled below their melt temperature.
Thermosetting resins or thermosets are those resins which, in the presence of a catalyst, heat radiation and/or pressure undergo an irreversible chemical reaction or cure. Prior to cure, thermosets may be liquid or made to flow under pressure and heat to any desired form. Once cured they cannot be returned to the uncured state and can no longer flow. One of the most common types of thermosetting materials are epoxy resins which are characterized by the presence of a three membered cyclic ether root commonly referred to as an epoxy group, 1,2-epoxide or oxirane. They have gained wide acceptance in composite materials because of their exceptional combination of properties such as toughness, adhesion, chemical resistance, and superior electrical characteristics. When combined with their relative ease of handling and processing as well as low unit cost, they make up the single most important matrix material.
In general epoxy resins can be cured by reaction with suitable, polyfunctional curing agents such as amines. The qualities of the curing agents in polymerization are governed by the structure and choice of components. For example, aliphatic amines allow ambient temperature curing whereas slow reacting, aromatic amines, require a higher temperature to cure. By varying and combining these curing agents, favorable production properties can be realized.
Thermoplastic systems have advantages over some of the thermosets in that no chemical reactions which cause release of gas products or excess thermal heat are involved. Processing is limited only by the time needed to heat, shape, and cool the structure. In addition they are generally more ductile and tougher than thermosets. On the other hand solvent resistance and heat resistance are not likely to be as good as with thermosets. Common thermoplastic materials include polyolefins, vinyls, polyamides, acrylics, polyesters, and polysulfones.
There are many processes for the fabrication of both thermosetting and thermoplastic composites. Such processes may be generally classified as open molding and closed molding. Open molds are one piece and use low pressure while closed molds are two piece and usually involve higher pressures. Closed molding techniques include matched die molding, injection molding, and continuous laminating. Finishing of the materials generally presents no major problems; the appropriate technology is both proven and cost effective. Rather, it is the preparation of composites in suitable form that tends to be costly.
Accordingly it is an object of this invention to provide an anisotropic reinforcement fabric which may be used in the fabrication of sturdy, lightweight, flexible composite structures.
Further it is an object of this invention to provide a sturdy lightweight composite material which may be easily formed into complex shapes.
In addition it is an object of the present invention to provide a process for the fabrication of sturdy, lightweight composite articles.
In particular it is an object of the present invention to form lightweight composite arch supports for use in shoes.